A wide variety of organic materials are present in artworks as paint binders, varnishes, pigments, coatings, and plastic materials, and are very often encountered as constituents of a wide range of archeological and historical objects, and as restoration materials. At the present state of the art, the characterization of organic materials in heritage objects is tackled mainly by non-destructive (and possibly non-invasive) spectroscopic techniques such as Raman and infrared spectroscopies, or by micro-destructive chromatographic and mass spectrometric approaches based on the identification of specific molecular markers [1,2,3,4,5,6,7,8,9,10]. As an alternative to sampling, attention has been focused on achieving mass spectrometric characterization by determining molecular markers or significant organic species in the volatile fraction emitted by the different materials.

In the last 10 years, the analysis of VOCs has been looked at also as a method for the identification of organic materials on the basis of the emitted volatile species [11,12,13,14,15]. The most common approach for the analysis of VOCs in museum environments is sampling by means of solid phase micro extraction followed by the gas chromatography mass spectrometry analysis (SPME-GC/MS) [16, 17].

VOCs analysis by this approach has also been exploited to investigate the occurrence of degradation processes of organic materials that are accompanied by emission of volatile species or by a variation in the molecular fingerprint of the emitted volatiles [14, 17,18,19,20,21]. The analysis of the VOCs emitted by beeswax [22], archeological resins [16, 23], and for inks [24] was also exploited for their characterization.

The main drawback of using SPME-GC/MS for the analysis of VOCs is that the determined VOCs profile strongly depends on the dynamic extraction and it is influenced by sorbent material, temperature, concentration, and qualitative and quantitative profiles of the VOCs in the sampled atmosphere.

To overcome the limitations related to selective SPME sampling and to implement VOCs analysis with the possibility to achieve in situ and real-time determination, attention has been recently drawn by methods of VOCs analysis based on portable in situ mass spectrometry. Selected ion flow tube-mass spectrometry (SIFT-MS) is a direct mass spectrometric technique, recently introduced as a portable device, which achieves real-time, quantitative analysis of VOCs in air at trace levels by applying precisely controlled ultra-soft chemical ionization, and eliminating sample preparation, pre-concentration and chromatography steps [25]. SIFT-MS employs different chemical ionization agents generated in situ to react with VOCs in controlled ion-molecule reactions, thus achieving detection limits in the parts-per-billion-by-volume (ppbv) range for most analytes, and even in the parts-per-trillion-by-volume (pptv) for specific species [26]. SIFT-MS has been extensively applied for the development of new clinic diagnostic methods [27,28,29,30,31], for quality control in food analysis [32,33,34,35], and for monitoring potentially dangerous security applications [36].

In this study, we explore for the first time the possibility to analyze natural and synthetic resins used as varnishes for artistic paints by means of SIFT-MS profiling of VOCs. In order to validate in situ analyses using SIFT-MS to identify varnish films in musealized objects, we set up proof of concept experiments based on the analysis of the headspace of films of the natural varnishes dammar, mastic, and colophony, and of the synthetic varnishes Paraloid B67, Regalrez 1094, PVAc, and MS2A. These resins were selected since they are amongst the most common materials used as varnishes and coatings in the field of cultural heritage.

Interpretation of SIFT-MS mass spectra by a qualitative point of view requires the knowledge of the significant markers in the VOCs profile, which were thus preliminary identified by SPME-GC/MS.

Materials and Methods

Varnishes

The mastic (triterpenoid resin, Kremer Pigmente, Germany), dammar (triterpenoid resin, Kremer Pigmente), Paraloid B67 (iso-butyl acrylate acrylic resin, CTS S.r.l., Italy), Regalrez 1094 (cyclohexane hydrocarbon resin, CTS S.r.l.), and polyvinyl acetate reference varnish layers selected for this study were cast in 2010 on glass slides, while colophony (distilled pine resin, Kremer Pigmente) and MS2A (cyclohexanone ketone resin, Kremer Pigmente) films were cast in 2018 from 25% (w/w) solutions (acetone for the natural resins and dichloromethane for the synthetic polymers). All the reference layers were left drying at room temperature in laboratory conditions and stored in the dark.

Selection of Volatile Compounds by SPME-GC/MS Analysis

Preliminary SPME-GC/MS analyses were performed using a StableFlex Divinylbenzene/Carboxen/PDMS fiber (Supelco, USA). The samples were inserted in the exposure chamber and left equilibrating at 60 °C for 60 min according to the literature [17, 37, 38]. The SPME fiber was than inserted in the exposure chamber for 30 min.

GC/MS instrumentation consisted in a 6890N gas chromatography system with a split/splitless injection port and combined with a 5973 mass selective single quadrupole mass spectrometer (Agilent Technologies, USA).

The fiber was desorbed in the GC injector port in splitless mode at 250 °C, with a split flow of 50 mL/min and a splitless time of 0.6 min. GC separation was performed on a fused silica capillary column HP-5MS (J&W Scientific, Agilent Technologies, stationary phase 5% diphenyl-95% dimethyl-polysiloxane, 30 m length, 0.25 mm i.d., 0.25-μm film thickness). Chromatographic conditions were initial temperature 40 °C, 1 min isothermal, 5.0 °C/min up to 250 °C, and isothermal 20 min. The helium (purity 99.9995%) gas flow was set in constant flow mode at 1.2 mL/min. MS parameters were electron impact ionization (EI, 70 eV) in positive mode, ion source temperature 230 °C, scan range 50–700 m/z, and interface temperature 280 °C. Peak assignment was based on a comparison with libraries of mass spectra (NIST 8 main EI MS library, WILEY 275 MS library).

SIFT-MS Analysis

The experiments were performed inserting the samples in a customized exposure glass chamber (Figure 1) which allowed both the insertion of the SIFT-MS probe and the SPME fiber. The SIFT-MS probe was inserted directly into the exposure chamber with an inlet flow rate of 25 mL/min. The accumulation time was optimized as described and discussed in the validation section. The full scan mass spectra of the natural and synthetic resins were acquired by a Voice 200ultra instrument (SYFT Technologies, New Zealand). The full-scan analyses were performed using the H3O+, NO+, and O2+ reagent ions, with nitrogen carrier gas. The product ions produced by the chemical ionization reaction of the reagent ions were monitored by a quadrupole mass spectrometer for a total acquisition time of 60 s.

Figure 1
figure 1

Scheme of the exposure chamber. The two openings are exploited alternatively, while the other is sealed by a glass cap

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The data acquisition and rates of reagent ion/resin VOCs reactions were obtained using the LabSyft 1.6.2 (SYFT Technologies). The interpretation of the mass spectra was performed by comparison with the data acquired by SPME-GC/MS and by manual comparison with the LabSyft Compound Library (SYFT Technologies). The product ions monitored for the quantitative analysis of α-pinene were m/z 137 (C10H17+) and m/z 155 (C10H16·H3O+) for the H3O+ reagent ion, m/z 92 (C7H8+) and m/z 93 (C7H9+) for the NO+ reagent ion, and m/z 93 (C7H9+) for the O2+ reagent ion [39].

Results and Discussion

Preliminary Analysis by SPME-GC/MS

Table 1 reports the VOCs identified by SPME-GC/MS for each material, while the chromatograms and the detailed VOCs list obtained for all the materials are reported in the Supporting Information (Figures S.1-S.7 and Tables S.1-S.7).

Table 1 Compounds Detected by SPME-GC/MS Analysis of the Vocs Emitted by the Investigated Paint Varnishes and Characteristic Ions Possibly Identified in the SIFT-MS Mass Spectra
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The SPME-GC/MS analysis on the VOCs emitted from the mastic resin allowed us to detect four groups of monoterpenes isomers characterized by molecular weights of 134 Da (C10H14), 136 Da (C10H16), 150 Da (C10H14O), and 152 Da (C16H10O) respectively, together with acetic acid and acetone [40].

The SPME-GC/MS profile of dammar featured acetone, acetic acid, and a group of sesquiterpenes characterized by a molecular weight of 204 Da (C15H24O) [41].

The analysis performed on colophony allowed us to identify a set of monoterpenes with a molecular weight of 136 Da (C10H14) and a set of sesquiterpene isomers with masses of 204 Da (C15H24), together with formic and acetic acids and acetone [42].

For what concerns synthetic resins, the VOCs profile of Paraloid B67 contained as its predominant compounds toluene, decane, and isobutyl methacrylate. Toluene is most probably the solvent used in the synthesis of the polymer.

The VOCs profile of MS2A was characterized by the presence of 4-methyl cyclohexanol and 1-butanol. Other minor peaks in the profile were assigned to acetic acid, 4-methylcyclohexane, and two dimers characterized by masses of 208 Da (C14H24O) and 210 Da (C14H26O) [43].

The chromatogram of Regalrez 1094 featured a series of methyl cyclohexane isomers with different aliphatic substituents in alpha position, with molecular weights of 126 Da (C9H18) and 140 Da (C10H20), and of a group of isomeric dimeric species with mass of 248 Da (C18H32) [43].

The SPME-GC/MS profile of the headspace of polyvinyl acetate reference mock-up featured the presence of acetic acid and benzene only.

The data obtained by the SPME-GC/MS analysis were used for the interpretation of the SIFT-MS results and mass spectra.

SIFT-MS Analysis

The headspace in the exposure chamber, after the accumulation time, was directly sampled through the SIFT-MS instrumentation. The mass spectra obtained for the natural and synthetic resins are reported in Figures 2 and 3 while the full list of the identified ions together with their assigned chemical formulas are reported in Table 1. The ions at m/z 37 and 55 using [H3O]+ as reagent ion are the result of association of H3O+ with H2O forming H3O+; those at m/z 30 and 48 using [NO]+ result from the association of NO+ and H2O; those at m/z 32, 37, and 55 using [O2]+ are a possible result of H3O+ minor impurities arising from the O2+ injection process.

Figure 2
figure 2

SIFT-MS mass spectra obtained for the natural varnishes mastic (a), dammar (b), and colophony (c) with different reagent ions; the reagent ions peaks are reported in gray

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Figure 3
figure 3

SIFT-MS mass spectra obtained for the synthetic varnishes Paraloid B67 (a), MS2A (b), Regalrez 1094 (c), and PVAc (d) with different reagent ions; the reagent ions peaks are reported in gray

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The SIFT-MS mass spectrum obtained for the mastic resin varnish using [H3O]+ as reagent ion (Figure 2a) was characterized by the presence of ions related to the monoterpenes identified by SPME-GC/MS analysis. In detail, the ions at m/z 137 and 135 can be related to the monoterpenes with a molecular weights of 136 Da and 134 Da after addition of H+ while the ions at m/z 151 and 153 correspond to the monoterpenes with molecular weights of 150 Da and 152 Da, respectively. Acetic acid and acetone were also identified thanks to the ions at m/z 61 and 59. By comparing the SPME-GC/MS results with the SIFT mass spectrum, the ions at m/z 117, 77, and 81 can be related to the induced fragmentation of the monoterpenes in the ionization flow tube in the presence of the reagent ion [44, 45].

The SIFT mass spectrum obtained with [NO]+ as reagent ion (Figure 2a) was characterized by the ions at m/z 136 and 166 deriving from the ionization of the monoterpenes with a molecular weight of 136 Da. The monoterpenes weighing 150 and 152 Da were also identified by the presence of the ions at m/z 150 and 152. As for the mass spectrum obtained by [H3O]+ used as reagent ion, the monoterpenes undergo a partial fragmentation by reacting with the reagent ions in the flow tube, leading to the formation of the ions at m/z 88, 92, and 93. Interestingly, no intense ions related to monoterpenes with a molecular weight of 134 Da were detected. Finally, the ions at m/z 59 and 61 can be related to the ionization of acetone and acetic acid.

The mass spectrum obtained for the resin with [O2]+ as reagent ion is more complex compared to the previous two reagent ions (Figure 2c). The mass spectrum is characterized by the presence of the ions at m/z 153, 151, 137, 60, 58, and 43 typical of monoterpenes, acetic acid, and acetone, similarly as discussed before. The mass spectrum was also characterized by the presence of the ions m/z 117, 93, 92, and 77, deriving from the induced fragmentation of the monoterpenes [44, 45].

The SIFT mass spectrum obtained for the headspace of dammar resin varnish using [H3O]+ as reagent ion (Figure 2b) was mainly characterized by the presence of the ion at m/z 205 related to the ionization of the sesquimers with a molecular weight of 204 Da, previously identified by SPME-GC/MS analysis. The mass spectrum featured also the presence of the ions at m/z 117 and 77, deriving from the fragmentation of the sesquiterpenes in the ionization flow tube in the presence of the reagent ions [39, 44, 45].

The SIFT mass spectrum obtained using [NO]+ (Figure 2b) was characterized only by the presence of the ion at m/z 204, related to the sesquiterpenes weighing 204 Da, and the ions at m/z 117 and 88 deriving from their fragmentation in the ionization flow tube in the presence of the reagent ions [39, 41, 42].

The [O2]+ reagent ion (Figure 2b) produced a much more complex mass spectrum with respect to the other two reagent ions, as observed for mastic, by inducing a fragmentation of the ions related to the sesquiterpenes with a molecular weight of 204 Da, detected along with the characteristic ions of acetone and acetic acid.

The mass spectra obtained after ionization with the three reagent ions (Figure 2c) were similar to each other. All spectra feature ions due to the ionization and induced fragmentation of the monoterpenes with a molecular weight of 136 Da, and of the sesquiterpenes weighing 204 Da. Moreover, the ions related to the presence of formic acid, acetone, and acetic acid were also detected in each mass spectrum.

The Paraloid B67 mass spectra obtained with [H3O]+ as reagent ion (Figure 3a) featured an ion at m/z 143 deriving from the ionization of isobutyl methacrylate, the monomer used for the production of the polymer, and an ion at m/z 93 related to the presence of toluene, solvent probably used in the synthesis of the polymers. The ion at m/z 161 is typical of the ionization of decane, when [H3O]+ is used as reagent ion. The identification of this species by SIFT-MS is particularly valuable because this information cannot be achieved by any spectroscopic non-invasive analysis.

The mass spectrum was also characterized by the presence of the ions at m/z 57 and 101 that can be related to the fragmentation of isobutyl methacrylate in the flow tube.

Similar results were obtained by using [NO]+ and [O2]+ as reagent ions (Figure 3a).

The mass spectrum obtained for the cyclohexanone resin MS2A using [H3O]+ as reagent ion (Figure 3b) is characterized by ions related to all the species detected by SPME-GC/MS analysis. In detail, the ions at m/z 97 and 131 were related to the ionization of 4-methyl cyclohexanol, while those at m/z 113 and 131 to the ionization of 4-methylcyclohexanone. The ions at m/z 189 and 227 can be ascribed to the presence of dimers with a molecular weight of 208 Da, while the ions at m/z 191 and 229 to the occurrence of dimers with a molecular weight of 210 Da. Moreover, 1-butanol was identified by the characteristic ions at m/z 57 and 75. Interestingly, the presence of the ion at m/z 93 cannot be easily related to any species identified by SPME-GC/MS; we can hypothesize that this ion derives from the induced fragmentation of the monomers and dimers of MS2A in the flow tube, in analogy to what occurs to the mono- and sesquiterpenes.

The spectra obtained using [O2]+ and [NO]+ as ionization reagents (Figure 3b) were characterized by the presence of all the ions characteristic of MS2A described above.

The mass spectrum of the aliphatic resin Regalrez 1094 obtained with [H3O]+ as reagent ion (Figure 3c) is mainly characterized by the ions at m/z 77 and 117 as the most abundant. The ions at m/z 127 and 141 corresponding to the monomers are detected at low intensities. The ion at m/z 117 can be associated to the ionization of methylcyclohexane using [H3O]+ as reagent ion. This information, along with the absence of ions related to the ionization of the dimers with a molecular weight of 248 Da, suggests that monomers and oligomers undergo an extensive fragmentation in the flow tube, leading to the formation of methylcyclohexane as a common specie. The same behavior was highlighted using [O2]+ as reagent ion, while the use of [NO]+ did not lead to any significant mass spectrum (Figure 3c), probably due to the low affinity of the monomers and oligomers of Regalrez 1094 with this ion.

The SIFT-MS mass spectrum of the polyvinyl acetate obtained with [H3O]+ as reagent ion (Figure 3d) featured the ions at m/z 61 and 78, associated with the ionization of acetic acid and benzene, respectively. The same analytical results were obtained using [NO]+ (m/z 78 and 90) and [O2]+ (m/z 43, 60, and 78) (Figure 3d).

In conclusion, all the species identified with the SPME-GC/MS analysis were also detected by SIFT-MS by identifying their specific ions.

SIFT-MS Method Optimization and Validation

In order to optimize the SIFT-MS approach, we selected mastic as a reference material for natural resins and Paraloid B67 for the synthetic ones. The molecular ions used for the study were m/z 137, due to α-pinene, for mastic; and m/z 143, characteristic of iso-butyl methacrylate. Blank spectra were also obtained when the resin was absent to ensure the origin of these marker product ions.

The optimization of the preconditioning time was performed by increasing the time from 5 to 60 min at 60 °C according to the literature [17, 37]. Figure 4 reports the plots of the ions’ intensities of the analytes together with the coefficients of variation obtained by triplicate analyses. Increasing the preconditioning time results in an enhancement of the signal of the two analytes, reaching a plateau after 45 min, along with a drastic decrease in the coefficient of variation. The best results for both classes of materials were obtained at 60 °C for 45 min.

Figure 4
figure 4

Ion intensities (Hz) and coefficient of variation (%) obtained at different accumulation times for mastic (a) and Paraloid B67 (b)

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It was possible to obtain the same intensity and coefficient of variation performing 12 h of preconditioning time at ambient temperature (25 °C). Thus, the use of an appropriate accumulation time in a museum environment, or by inserting the probe in microclimate frames, could allow one to obtain reliable results without heating the sample.

The interday and intraday relative standard deviations (RSDs) were also evaluated by comparing the ions’ intensities: for both reference materials, the intraday RSD% was lower than 9% and the interday RSD% lower than 17%.

In order to evaluate the linearity of the response with respect to the amount of material contained in the exposure chamber, we tested the instrument in the quantitative analysis mode. For this purpose, we selected α-pinene in mastic, since it is one of the analytes present in the LabSyft Compound Library, for which the absolute quantitative analysis is possible. The experiments were performed by analyzing decreasing amounts of aliquots of the reference material placed in the exposure chamber, in the range 0.1–1.0 mg. A significative concentration of α-pinene was easily detectable in all samples, and the quantitative approach showed a good linearity with a R2 of 0.9914.

Conclusions

In this paper, we present the first analytical study exploring the possibility to apply SIFT-MS to characterize the organic materials used in artworks or as restoration varnishes. We tested SIFT-MS transportable analytical instrumentation on 7 different reference resins, both natural and synthetic, to evaluate acquisition of acceptable qualitative data for the characterization of these materials.

By using the SIFT-MS approach, all the natural and synthetic resins were characterized by specific and characteristic mass spectra. Moreover, the identification of the material is still possible even in the case of extensive fragmentation of the molecular markers. An example is Regalrez 1094 whose mass spectra are dominated by the ion at m/z 117 deriving from the extensive fragmentation of the monomers and dimers, but also contain ions at 127 and 141 typical of the monomers.

The promising results obtained in this survey represent the first step in the development of a completely new analytical approach for the non-invasive/non-destructive characterization of organic materials. The advantages in the use of this instrumentation, such as the high selectivity, and the possibility to perform in situ analysis, could be extremely relevant within cultural heritage where the characterization of organic materials requires novel non-invasive analytic approaches.

The possibility to interface this portable SIFT-MS instrumentation to microclimate frames could be a powerful combination for the characterization of the organic materials through the analysis of the VOCs profile. Moreover, the development of exposure chambers with different dimensions suitable for specific artworks or even archeological objects could allow a wider range of applications.

Finally, this new analytical approach could be an extremely powerful tool in evaluating the VOCs emitted by varnishes or solvents during the restoration processes, to contribute both in the monitoring of the progresses of the work and in ensuring the health and safety of the restorers.